This invention relates generally to items made from metallic powders and relates, more particularly, to the formation and treatment of porous items comprised of powdered stainless steel or any alloy which forms an oxide surface, such as chromium oxide, aluminum oxide, or silicon oxide, on oxidation.
Porous items constructed of powdered metals and with which this invention is concerned are commonly utilized in applications which rely upon the porous nature of the item for the item to operate in its intended manner. Such items can include, for example, filters, membrane supports or substrates, and fuel cell supports and can be formed with processes involving molding, extrusion, casting or isostatic compression. Heretofore, however, when such an item is used or undergoes processing at relatively high temperatures, i.e. within 200 and 300 C. degrees of the melting temperature of the material which comprises the item, the item becomes non-porous (or its porosity closes, i.e. it becomes no longer interconnected) and thereafter cannot operate in its intended manner or continue to be processed. Such a loss of porosity in the item is due, at least in part, to the exposure of the relatively large surface areas possessed by the item to the relatively high temperatures.
Some stainless steels, and in particular Series 400 stainless steels, have melting temperatures which fall within the range of between about 1370° C. and 1530° C. depending upon the specific composition of the steel in this class. Therefore, an item formed with a conventionally-processed stainless steel of this class is likely to experience a loss in porosity when exposed to a temperature as low as about 1200° C.
It would therefore be desirable to provide a method for treating a porous metallic body which enhances the thermal stability of the body when exposed to temperatures which approach the melting temperature of the material which comprises the body.
Accordingly, it is an object of the present invention is to provide a new and improved method for processing a porous metallic item which imparts to the item a thermal stability which prevents the item from losing its porosity when exposed to temperatures which are within about 200 and 300 C. degrees of the melting temperature of the item.
Another object of the present invention is to provide such a method which enables the processed item to retain many of its desirable metallic properties, such as its ductility, at these relatively high, near-melting temperatures.
Still another object of the present invention to provide such a method which is particularly well-suited for processing a porous item comprised of stainless steel, and in particular, Series 300 and Series 400 stainless steel, or an alloy that forms a surface oxide, such as chromium oxide, aluminum oxide, or silicon oxide, on oxidation.
Yet another object of the present invention is to provide such a method which is uncomplicated to perform, yet is effective in operation.